Fat can accumulate in the lungs of people, particularly those who are overweight or obese, researchers have shown for the first time.
Scientists already know that people who are overweight or obese are more likely to suffer with wheezing and asthma, but the reasons for this have not been completely explained.
The new study, published in the European Respiratory Journal, suggests that this fatty tissue alters the structure of people's airways and this could be one reason behind the increased risk of asthma.
"Looking at samples of lung, we spotted fatty tissue that had built up in the airway walls. We wanted to see if this accumulation was correlated with body weight," said John Elliot, a senior research officer at Sir Charles Gairdner Hospital in Australia.
The researchers examined post-mortem samples of lung that had been donated for research and stored in the Airway Tissue Biobank.
They studied samples from 52 people, including 15 who had no reported asthma, 21 who had asthma but died of other causes and 16 who died of asthma.
Using dyes to help visualise the structures of 1373 airways under a microscope, they identified and quantified any fatty tissue present. They compared this data with each person's body mass index (BMI).
For the first time, the study showed that fatty tissue accumulates in the walls of the airways.
The analysis revealed that the amount of fat present increases in line with increasing BMI.
The research also suggests that this increase in fat alters the normal structure of the airways and leads to inflammation in the lungs.
"Being overweight or obese has already been linked to having asthma or having worse asthma symptoms," said Peter Noble, an associate professor at the University of Western Australia.
"Researchers have suggested that the link might be explained by the direct pressure of excess weight on the lungs or by a general increase in inflammation created by excess weight," Noble said.
This study suggests that another mechanism is also at play.
"We've found that excess fat accumulates in the airway walls where it takes up space and seems to increase inflammation within the lungs.
"We think this is causing a thickening of the airways that limits the flow of air in and out of the lungs, and that could at least partly explain an increase in asthma symptoms," Noble said.
The researchers are looking for new ways to study and measure fatty tissue in the lungs.
They want to confirm the relationship with respiratory disease and to find out whether the effect can be reversed by weight loss therapy.
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